1. Field of the invention
The present invention relates to an automatic transmission which is installed in, for example, a front-engine front-drive vehicle and which changes the input speed received through a first shaft, through operation of an automatic speed change gear mechanism having a planetary gear unit and engaging elements, and outputs the changed speed through an output gear located in a central portion of the automatic speed change gear mechanism and through a second shaft to a differential mechanism. More particularly, the present invention relates to an automatic transmission wherein the inter-shaft distance and the axial length of the second shaft are reduced to provide a more compact transmission and to facilitate installation in a vehicle.
2. Description of the Related Art
Various automatic transmissions have been developed for vehicles in which an automatic speed change gear mechanism includes a planetary gear unit having a plurality of rotary elements and further includes a plurality of frictional engagement elements, i.e. clutches and brakes, for selectively engaging the rotary elements. Speed change control is by controlling engagement and disengagement of the frictional engagement elements to, in turn, control rotation of the elements of the planetary gear unit.
An example of such a conventional automatic transmission is disclosed in U.S. Pat. No. 5,224,904, wherein the input speed, i.e. speed of a first shaft, is changed by an automatic speed change gear mechanism having a planetary gear unit and frictional engagement elements. The output is extracted at an output gear located substantially central to the first shaft, and transmitted by a second shaft f to a third shaft of a differential device.
FIG. 12 is a skeletal diagram schematically illustrating an automatic transmission A disclosed in the aforementioned U.S. Pat. No. 5,224,904.
In the automatic transmission A illustrated in FIG. 12, when one of the P, R, N and D ranges is selected using a manual shift valve (not shown), clutches C0-C3, brakes B1-B2 and one-way clutches F0-F1 of an automatic speed change gear mechanism a become engaged or disengaged, as shown in FIG. 13, responsive to a combination of on-off signals from various solenoid valves in a hydraulic control circuit (not shown), corresponding to the selected and set range. Rotation of the rotary elements of a planetary gear unit b, such as a sun gear, a ring gear, pinion gears, a carrier and the like, is thereby controlled so that the engine drive force input to a first shaft (input shaft) d, through a torque converter c, is changed in speed. The speed-changed rotation is output from a counter drive gear e disposed in a central portion of the automatic transmission A. In FIG. 13, 0 indicates engagement and X indicates disengagement.
The output from the counter drive gear e is transmitted to a counter driven gear g on a second shaft f, and transmitted from the second shaft f to a third shaft of a differential device h, that is, to the right and left differential shafts i, j.
Recently, the demand has grown for a more compact automatic transmission and improved ease of vehicle installation. The engine compartment of a motor vehicle provides a limited, relatively small space, in which an engine and various other devices and components are mounted along with the automatic transmission. Therefore, even a slight size reduction of an automatic transmission is now of great significance.
The automatic transmission A disclosed in the aforementioned U.S. Pat. No. 5,224,904 employs a design as described below to enable size reduction. As specifically shown in FIG. 14, a parking gear k mounted forward (right side in FIG. 14) of the counter driven gear g on the second shaft f is positioned radially outward of the outer peripheries of a clutch C2 and a brake B1 which are mounted on the first shaft d at the engine side, that is, forward of the counter drive gear e of the automatic speed change gear mechanism a, and parking gear k axially overlaps the clutch C2 and the brake B1. By disposing the counter drive gear e in substantially the center of the automatic speed change gear mechanism a in this manner, a further reduction of the axial length of the second shaft f carrying the counter driven gear g, which engages the counter driving gear e, is achieved.
Furthermore, by disposing the clutch C2 and the brake B1 at the engine side of the counter drive gear e, i.e. on an engine-side portion of the first shaft d, the space m defined between the engine side of the counter drive gear e and a torque converter (T/C) 6, is effectively used.
With such an arrangement of the parking gear k, the clutch C2 and the brake B1, the axial dimensions of the first shaft d and the second shaft f can be reduced, and the size of the automatic transmission A correspondingly reduced.
However there is intense demand for a further size reduction of automatic transmissions, even for the "compact" transmissions, such as the aforementioned automatic transmission A. For a further size reduction of the automatic transmission A, it is conceivable to reduce the inter-shaft distance between the first shaft d and the second shaft f. However, such relocation of the second shaft f closer to the first shaft d will result in interference of the parking gear k with the clutch C2 and with the brake B1 because of the design in which the parking shaft k on the second shaft f is disposed under the clutch C2 and the brake B1 on the first shaft d, that is, the parking gear k axially overlaps the clutch C2 and the brake B1. Further, the spacing between the first shaft d and the second shaft f must be at least the sum of the radius of the brake B1 and the radius of the parking gear k, while, as a practical matter, minimum radii of the clutch C2, the brake B1 and the parking gear k are fixed. Therefore, there is a limitation on the extent to which the second shaft f may be relocated closer to the first shaft d and that limitation is the sum of the radius of the brake B1 and the radius of the parking gear k. Thus, it is almost impossible to further reduce the inter-shaft distance between the first shaft d and the second shaft f.
To avoid interference of the parking gear k with the clutch C2 and the brake B1, the parking gear k can be disposed in the axial space between the counter drive gear e and the clutch C2 by simply by relocating the counter driving gear e, the frictional engagement elements other than the clutch C2 and the brake B1, and the planetary gear unit b axially rearward. It thereby becomes possible to dispose the second shaft f considerably closer to the first shaft d. However, such a modification would require an increase in the lengths of the first shaft d and the second shaft f in the axial direction, and therefore would fail to reduce the overall size of the automatic transmission A, despite a reduction in the inter-shaft distance between the first shaft d and the second shaft f. Moreover, reverse clutch C2 which engages for reverse driving, requires a relatively large capacity and a relatively great length in the axial direction. Therefore, if the parking gear k is disposed in an axially extending space formed between the counter drive gear e and the clutch C2 as described above, the counter drive gear e must be moved further rearward. Since the counter driven gear g which meshes with the counter drive gear e must be disposed a corresponding distance rearward, the second shaft f must be further elongated.
For the foregoing reason, the automatic transmission A disclosed in the aforementioned U.S. patent cannot be said to fully meet the demand for size reduction in an automatic transmission.